1. Field of the Invention
This invention concerns an oil recovery process and more specifically is concerned with a method for predicting the amount of chemicals retained within a subterranean reservoir during the course of an oil recovery process.
2. Description of the Prior Art
The crude oil which has accumulated in subterranean reservoirs is recovered or produced through one or more wells drilled into the reservoir. Initial production of the crude oil is accomplished by a primary recovery technique wherein only the natural forces present in the reservoir, such as gas drive and natural water drive, are utilized to produce the oil. However, upon depletion of these natural forces and the termination of primary recovery, a large portion of the crude oil remains trapped within the reservoir. Also, many reservoirs lack sufficient natural forces to be produced by primary methods from the very beginning. Recognition of these facts has led to the development and use of many enhanced oil recovery techniques. Most of these techniques involve injection of at least one fluid into the reservoir to produce an additional amount of the crude oil therefrom.
Water flooding involves injection of water into the subterranean oil reservoir for the purpose of displacing the crude oil from the pore spaces of the reservoir rock towards the producing wells. It is the most economical and widely used of the enhanced oil recovery methods. Nevertheless, water does not displace oil with high efficiency because of the high interfacial tension between water and oil and because of the resulting immiscible displacement of oil by water.
Because of the inherent low efficiency of the basic water flooding method, the petroleum industry has for many years sought additional chemicals, which when added to a water fluid, will increase the efficiency of the water flooding method. A few of the chemicals which have been found useful for this purpose are surfactants, solubilizers, polymers, sacrificial agents, caustic additives and other reservoir conditioning agents.
The greater efficiency achieved by the addition of these chemicals to a water fluid is offset by the high cost of the chemicals themselves. In order for a petroleum recovery operation to be economically justifiable the value of the petroleum recovered by the process must, of course, exceed the cost of the recovery process itself. It is also known that most, if not all, of these chemicals are, to varying degrees, retained within the reservoir rock and are not to any large extent recoverable during the course of the petroleum recovery operation. To this end there is a substantial need to be able to accurately predict the amount of any such chemical that will be retained within the reservoir rocks during the course of the petroleum recovery operation. Knowledge of such an amount is crucial to the design of the chemical flood program.
Several different types of methods have been proposed for determining the chemical requirements for a chemical flooding program in which the amount of the chemical retained within the formation is determined. Such methods range in complexity from small scale laboratory bench testing of core materials to large scale multiwell pilot tests in the field. Unfortunately laboratory data are often unable to accurately predict chemical retention values under reservoir conditions due to the difficulties involved in translating information obtained from core flooding tests into information that is applicable to the immense heterogeneous reservoir rock volumes that comprise a typical petroleum reservoir. On the other hand, while a large scale multiwell pilot testing program will usually be able to provide fairly accurate chemical retention data, the costs involved in both expense and time in such programs are often prohibitive. In between these two methods fall methods involving only a single well to determine chemical retention data, bridging the gap between the laboratory and pilot floods. This type of procedure is attractive because a sufficiently large volume of reservoir is contacted to give meaningful results, and the results are usually able to be obtained within reasonable time and cost limits. One such technique is disclosed in the June 1967 issue of the Journal of Petroleum Technology in a paper by H. R. Froning and R. O. Leach entitled "Determination of Chemical Requirements and Applicability of Wettability Alteration Flooding." Nevertheless, although the method has appeared promising, the methodology employed to extrapolate the data obtained from these single well test methods to the reservoir as a whole has been suspect and there remains a present need for a method which will produce accurate, reliable results.